1. Technical Field of the Invention
The present invention relates to integrated circuits, especially to memory circuits and in particular to static random access memory (SRAM memory) cells. More particularly, the invention relates to limiting the leakage currents in the transistors for accessing the static memory (SRAM memory) cells.
2. Description of Related Art
Memory cells, and in particular SRAM memory cells, are produced in the form of a matrix of memory cells that are arranged in rows and columns of memory cells and are connected in the differential mode between two bit lines. During a read operation, one of the bit lines after having been precharged to a high voltage is discharged, that is to say taken to the ground potential, whereas the other line is assumed to remain at its high precharge level.
FIG. 1 shows a conventional static memory cell.
As may be seen in this figure, the cell C consists of two inverters I1 and I2, which store one bit, and of two access transistors T1 and T2 via which the cell is connected to two complementary bit lines BL and BLB, which serve to read the memory location or to modify it. The input and the output of one of the inverters I1 communicate with the output and the input of the other inverter I2, respectively, and constitute nodes N1 and N2 respectively, the voltage levels of which correspond to the value of a stored bit. The two transistors are controlled by a word line WL for transferring the stored bit to the bit lines BL and BLB during a read operation or to impose the state of the lines BL and BLB to the memory location during a write operation.
In particular during an operation of reading a logic state 0 (and vice versa 1), the bit line BLB (and vice versa BL) precharged beforehand to a high voltage Vdd is discharged, whereas the bit line BL (and vice versa BLB) precharged beforehand to a high voltage Vdd is assumed to remain at its high level. Under the command of the access transistors T1 and T2, the nodes N1 and N2 are then positioned at 0 and at Vdd respectively (or vice versa at Vdd and at 0). However, leakage currents appear through the access transistors, and in particular in the access transistor T1 that connects the cell to the bit line BL maintained at its high precharge level.
Since a memory integrated circuit comprises a very large number of rows of memory cells, the sum of the leakage currents flowing through the access transistors for the set of rows results in a not insignificant modification in the level of the bit line BL that is assumed to remain in the high state (in the case in which the stored value corresponds to a logic 1), until a level from which the voltage differential between the two bit lines is no longer sufficient to ensure correct operation of the memory location, despite the presence of read amplifiers used to amplify the voltage difference between the two lines BL and BLB.
Consequently, the leakage currents created in the access transistors set a limit on the number of memory cells per bit line so as to avoid the risk of losing stored information.
In the state of the art, it has been proposed to alleviate this drawback by grouping memory locations together in the form of blocks of memory locations each connected to the bit line via a transfer port.
FIG. 2 shows the implementation of such a matrix of memory cells.
For the sake of clarity, only two columns of memory locations, including two bit lines BL0, BLB0 and BL1, BLB1, have been shown.
As may be seen in this figure, the memory locations, represented by their access transistors T and T′, are grouped together in the form of blocks B1, B2, . . . , Bn that communicate with the bit lines BL0, BLB0 and BL1, BLB1 via transfer ports, such as P.
For the purpose of limiting the discharge of the overall bit lines, the number of memory locations in each block is limited to about 256 memory locations. Thus, even if leakage currents do appear in all the access transistors of one of the groups, the cumulative discharge generated in the bit lines remains insufficient to result in an overall discharge liable to cause a loss of information.
However, the production of memory locations requires very stringent fabrication constraints. Such constraints cannot be applied to the production of the transfer ports without incurring an unacceptable increase in the fabrication costs.
Furthermore, producing memory locations in the form of separate blocks results in discontinuities within the matrix, insofar as the memory locations lying in a heterogeneous environment are increased. Consequently, there is reduced matrix integration.
What is more, the relative slowness of the transfer ports impairs the overall performance of the matrix. Furthermore, these ports require their control to be synchronized with the control of the other ports and with the access transistors.
In the light of the foregoing, there is a need in the art to alleviate the foregoing drawbacks and, in particular, to provide a memory circuit and a corresponding fabrication process that limit the discharge of the bit lines to an acceptable level, without limiting the number of cells connected to the bit lines, while reducing matrix discontinuities.